A computer network (e.g., Internet) involves data exchange between a terminal device (e.g., a laptop, a tablet computer, or a smart phone) and a network server (e.g., a web server). The terminal device sends out an access request to access a piece of day (e.g., a web page) hosted by the network server. A domain name sever (DNS) receives the access request, determines the internet protocol (IP) address of the network server, and forwards the access request to the network server. The network server returns information of the accessed web page to the terminal device upon receiving the access request. In many cases, the source code of the web page may have security loopholes due to, e.g., lack of experience of the person who develops the source code. As such, the network server may be vulnerable to malicious attacks.